Numerous methods have become known for detecting combustion misfires. Of these, only those are of interest in the following which operate with so-called uneven-running values. Values of this kind are essentially a measure for rapid changes of the crankshaft rpm of the engine. As a rule, uneven-running values are specified with respect to sign in that a rapid drop of rpm leads to a large positive uneven-running value for the affected cylinder whereas the uneven-running values of normal combustions are significantly less and are possibly negative. This rapid drop of rpm can be caused by a combustion misfire. If the actual uneven-running value for a cylinder exceeds a threshold value, which is typically read out of a characteristic field in dependence upon actual values of operating variables, then this is a sign for a misfire in the particular cylinder. When forming uneven-running values, an opposite sign application can be used in an equivalent manner in the sense that a negative uneven-running value corresponds to a rapid reduction of rpm. In this case, a misfire is detected in that the corresponding uneven-running value drops below a negative threshold value.
An overview of various methods and arrangements for detecting combustion misfires is provided in DE-A-40 09 285 and in DE-A-41 38 765. Reference is made hereto for the sake of brevity. In the present invention, it is not significant as to how the uneven-running values are formed; instead, what is of concern is how, with the aid of any desired determined uneven-running values, a reliable decision can be made as to whether misfires are present or not.
It is problematic in the detection of misfires that not only misfires lead to rpm fluctuations but that such fluctuations are also transmitted to the engine by vibrations of the vehicle drive train. Such vibrations are especially caused by unevenness of the road surface.
Road surface unevenness can be determined with the aid of the signals of an acceleration sensor on the affected vehicle or with the aid of a signal which monitors fluctuations of the rpm of a wheel. If the determined values exceed a pregiven threshold value, concern can be had that the detection of misfires is influenced disadvantageously and for this reason the misfire detection is interrupted (preferably the computation of uneven-running values is already interrupted) until the above-mentioned signals again drop below the particular threshold value. Corresponding methods and arrangements are however, tied to increased complexity and increased costs.